The present invention relates to a superconductor made of a plurality of superconductive filaments which are embedded in normal conducting material. More particularly, the present invention relates to such a superconductor wherein the filaments are arranged concentrically in the cross section of the superconductor and are twisted about the longitudinal axis of the conductor, each filament forming a helix which is concentric with the longitudinal axis. The result of this arrangement of the filaments is the suppression of coupling effects between the filaments in circles of different radii, which are produced as a result of time varying magnetic fields.
Superconductors of this type, i.e., of the multifilament wire type, are required with increasing frequency to produce coils for multipole magnets, e.g., for particle accelerators in high energy physics and for coil arrangements for the enclosure of plasma in fusion experiments.
These superconductors operated in pulsed magnetic fields of a high intensity, with a field component having a transverse orientation, i.e., perpendicular to the longitudinal axis of the conductor, and a field component having a longitudinal orientation, i.e., parallel to the longitudinal axis of the conductor.
It is known (Journal of Applied Physics, Vol 40, pages 2080-2082, 1969) to employ multifilament wire type superconductors for conducting electrical current in time variable magnetic fields. These conductors consist of a plurality of filaments of superconducting material such as NbTi with a diameter of a few microns, which are embedded in a matrix of normally conducting material, such as copper, and form a multifilament wire.
The filament diameter is kept as small as possible (5 to 50.mu.) in order to substantially prevent premature changeover to normal conduction before reaching the critical current (magnetic stability) and reduce a.c. field losses.
In time variable magnetic fields which are oriented transversely to the conductor axis, the filaments are coupled by induction effects. Thus, above a predetermined rate of change of field, the superconductor will behave as if it were made of but a single superconductive core with all the attendant drawbacks with respect to magnetic stability and a.c. field losses. It is also known that, to overcome these drawbacks, the superconductor can be twisted about its longitudinal axis (Journal of Physics D 3, pages 1517-1531, 1970).
This twisting of the multifilament wire, which is of advantage for the case of time variable transverse magnetic fields, has the drawback that with the presence of time variable longitudinal magnetic field components, i.e., components which are oriented parallel to the conductor axis, new coupling effects will occur. Filaments arranged at two different radii within the conductor form coils having the same number of windings but different winding areas with respect to a longitudinal field component extending in the direction of the conductor axis. Therefore, a temporary change in the magnetic field produces voltage differences leading to coupling currents between these two coils which currents become greater with increasing conductor length and decreasing length between adjacent coil turns, i.e., increasing number of windings in the coils. Above a predetermined rate of change of field these coupling currents lead to shielding of the magnetic field and thus to magnetic instability and higher a.c. field losses.